Abstract
Background
The AIBL study of ageing is an experimental prospective study of 1112 elderly Australians at risk of, or clinically diagnosed with, Alzheimer's disease (AD). The AIBL cohort has now completed the fifth assessment, 72 months after enrolment. Here we report rates of attrition, disease progression and mortality for this assessment.
Methods
The inclusion/exclusion criteria and assessment protocol for the AIBL cohort has been described in detail elsewhere (Ellis et al., 2009). Briefly, participants undergo an array of blood tests, clinical and cognitive evaluation and brain imaging (in a subgroup) every 18 months. At the completion of assessment, all data is integrated and reviewed by an expert panel of psychiatrists, neurologists and neuropsychologists where a consensus clinical classification is made. At baseline, 768 (69.06%) participants were classified as cognitively normal (CN); 133 (11.96%) as meeting clinical criteria for Mild Cognitive Impairment (MCI) and 211 (18.97%) for AD.
Results
598 participants (53.78% of the initial cohort) were re-assessed at 72 months. 119 (10.70%) had deceased and 395 (35.52%) failed to return (FTR). 484 participants (80.94%) were classified as CN; 49 (8.19%) as MCI; 59 (9.87%) as AD and 7 (1.17%) as having non-AD cognitive impairment or dementia. Of the baseline CN group, 26.30% FTR, 11.20% progressed to MCI or AD and 2.47% died. Of the baseline MCI group, 51.88% FTR, 15.79% progressed to AD and 16.54% died. Of the baseline AD group, 49.29% FTR and 35.55% died. Survival analyses indicated clinical groups were equivalent in rate of attrition due to death or FTR, and in rates of mortality with FTR controlled. With mortality and FTR controlled, rate of progression to AD was greater in the MCI group than the rate of progression to MCI or AD in the CN group (p<0.01).
Conclusions
The AIBL study has successfully followed over 50% of its initial cohort across 6 years. While group membership at baseline does not predict death or FTR, rates of disease progression are higher in the MCI than CN group. The AIBL study remains as a unique cohort that still contains sufficient sample size for investigation of risk factors for AD.